In recent years, organic EL displays using electroluminescence (EL) of an organic material have been attracting attention as a type of next-generation flat panel display replacing liquid crystal displays.
Unlike the voltage-driven liquid crystal display, the organic EL display is a current-driven display device. Accordingly, there is an urgent need for development of a thin-film transistor (TFT) having excellent characteristics as a driving circuit for the active-matrix display device. The TFT is used as a switching device for selecting a pixel or a driving transistor for driving the pixel.
A description is given of a configuration of a conventional thin-film semiconductor device (a thin-film transistor), with reference to FIG. 7 (for example, see Patent Literatures (PLT) 1 and 2). A thin-film semiconductor device 900 shown in FIG. 7 includes: a substrate 910; a gate electrode 920; a gate insulating film 930; a crystalline silicon layer 940; a non-crystalline silicon layer 950; a channel protective layer 960; a pair of contact layers 971 and 972; a source electrode 981; and a drain electrode 982. The thin-film semiconductor device 900 is a bottom-gate thin-film transistor formed by stacking these elements in the mentioned order.
In the thin-film semiconductor device 900 with such a configuration, positive fixed charge exists in the channel protective layer 960. Hence, a back channel is formed in the crystalline silicon layer 940 which includes a channel region, due to the fixed charge. As a result, leakage current occurs, degrading the off-characteristics. Here, the back channel refers to a pathway of parasitic current which flows from the source electrode 981 to the drain electrode 982 via the vicinity of the interface, within the crystalline silicon layer 940, between the crystalline silicon layer 940 and the channel protective layer 960.
In view of this, the non-crystalline silicon layer 950 made of an amorphous silicon film is formed between the crystalline silicon layer 940 and the channel protective layer 960. The non-crystalline silicon layer 950 can shield the electric field by cancelling positive fixed charge in the channel protective layer 960 by the charge density of the negative carriers. With this, it is possible to suppress formation of a back channel, suppressing leakage current when the thin-film semiconductor device 900 is turned off. Accordingly, the off-characteristics can be improved.